Splenic B cells of young and aged rats were separated in a FACS into a population of brightly fluorescent cells with high density of surface Ig (SIg) (13%) and a population of poorly stained lymphocytes with low density of SIg (80%). The latter population of both young and aged rats yielded 3-5 times more PFC after immunization with sheep Rbc than did the brightly stained population; and spleen cells of aged rats yielded 10-30% of PFC generated from spleens of young rats, in both dull and bright staining populations. Capping of SIg was significantly less and slower in brightly fluorescent cells than in poorly stained cells. by light scatter analysis, the size of splenic B cells of aged rats was larger than the size of young splenic B cells. The T antigens Thyl.l, W3/13 and W3/25 of aged rat lymphocytes, after reaction with two antibodies, cap significantly slower than in young rat lymphocytes, and "aged" T lymphocytes have less density of these antigens on their surface than do "young" T cells. Capping rates of these antigens are fastest in thymus cells, less in lymph node cells, least in spleen cells, from both young and old rats. Colchicine facilitates capping of W3/13 in spleen cells of young rats, after reaction with 1 degree antibody but does not effect capping in spleen cells of old rats. Thymidine uptake of "aged" T lymphocytes in response to Con A and PHA is reduced to 20-30% of the response observed in splenic T cells of young rats. Membrane molecules aggregate in the cell membranes of lymphocytes according to polarity of the field, when cells are subjected to 4-8V/cm field charge. SIg, W3/13 and Con A receptors move to the negative pole; Thyl.l moves to the positive pole; Ia moves to negative and positive poles; and W3/25 displays no movement. Motility is directed toward the positive pole following use of a field charge.